Among business machines which have made a marked step forward, printers have changed from the wire dot and thermal transfer printing systems to the ink jet and laser beam printing systems. In-the ink jet printing system, an ink retainer is typically inserted in a container in order to retain the ink uniformly in the container.
Without such an ink retainer, it is difficult to feed ink from the container at a constant rate because the feed rate of ink can vary with the residual amount of ink in the container. In the printing system wherein the ink container moves together with the printing head, the ink always waves and sloshes in the container, hindering a constant ink supply and generating noise due to the collision of ink waves against the container wall.
For this reason, ink retainers are essentially needed in ink containers and are generally made of flexible polyurethane foams. Where an ester type polyurethane foam is used as an ink retainer, the foam itself can be deteriorated with a particular type of ink and such decomposition products can later be leached into the ink to alter the ink composition. For example, this results in ink having reduced surface tension so that letters printed therewith may be blurred. Usually polyurethane foam has cell membranes. If the foam is inserted in the ink container without removing the cell membranes, the cell membranes can adversely affect or alter the capacity to retain ink and the ability to release ink.